The pygidium is the posterior body part or shield of
crustaceans and some arthropods, such as insects and the extinct trilobites. It contains the anusand, in females, the ovipositor. It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an articulation.
arachnids, the pygidium is formed by reduction of the last three opisthosomal segments to rings where there is no distinction between tergites and sternites. A pygidium is present in Palpigradi, Amblypygi, Thelyphonida, Schizomida, Ricinuleiand in the extinct order Trigonotarbida. It is also present in early fossil representatives of horseshoe crabs.
"Pygidium" is also a superseded
genusof fishof the family Trichomycteridae. Most species of this genus have been reassigned to the genus " Trichomycterus".
In trilobites, the pygidium can range from extremely small (much smaller than the head, or
cephalon) to larger than the cephalon. They can be smooth, as in order Asaphida, or spiny, as in order Lichida.
Shultz, J.W. (1990). Evolutionary Morphology And Phylogeny of Arachnida. Cladistics 6: 1-38.
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